Many electrical circuits in a wide range of technical fields rely on inductive elements, such as solenoids, electromagnetic valves, contactors, relays or electric motor drives, driven as a load for their operation.
With any circuit containing an inductive load, however, a problem occurs when the current through the load is switched off. Since the voltage across an inductor is proportional to the rate of change of inductor current, then any attempt to rapidly switch off the load current results in a large voltage spike, which can damage or destroy components in the driving circuit. This problem is due to the energy stored by the load current within the magnetic field of the inductor being returned abruptly to the inductor coils as the magnetic field collapses. In conventional circuits containing an inductive load, the energy contained within the magnetic field is dissipated as heat by using a circuit such as a snubber network or a catch diode that diverts the voltage spike to a reference voltage, usually ground.
There is an additional problem with conventional circuits employing inductive loads that exhibit limited mechanical movement, such as solenoids, electromagnetic valves, contactors or relays. When such circuits are switched on, the energising current needs to be such that the magnetic field quickly rises to a level to create the initial pull; however, if the current is not reduced quickly at this time, then power loss is incurred as heat as the movement of the device is abruptly halted.
In battery-powered equipment or any device that must run at low powers or low temperatures, this wasted energy and the heat it causes in both of the above examples can be a significant problem.
A review of inductive drive circuits is given in "Noise Reduction Techniques in Electronic Systems", 2nd edition by Henry W. Ott, published by John Wiley & sons, 1988, pp216-223.
This invention seeks to provide an improved switching circuit for an inductive load that mitigates the above mentioned disadvantages.